This invention relates to the production of ammonium polyphosphate solutions. More particularly, it relates to the production of stable ammonium polyphosphate fertilizer solutions by the direct ammoniation of concentrated phosphoric acid derived from wet-process acid. Wet-process phosphoric acid is produced by decomposing phosphate rock, particularly calcium phosphate, with sulfuric acid and separating the phosphoric acid from the precipitated calcium sulfate. The resulting crude phosphoric acid contains various amounts of impurities such as iron, aluminum, calcium, magnesium and other metals in the form of their phosphates, sulfates or fluorides, depending on the characteristics of the particular phosphate rock which is used as a starting material. Because of the simplicity and cheapness of its manufacture, wet-process phosphoric acid is regarded as an excellent source of fertilizer grade phosphates. However, because of the presence of the aforementioned impurities its use is fraught with many difficulties and complications. As is well known, when wet-process phosphoric acid is treated with ammonia to form aqueous ammonium phosphate solutions, the impurities present in the acid form gelatinous precipitates which are extremely difficult to separate from the aqueous phase by filtration or other conventional methods. These precipitates form large deposits in the equipment used for storage and transfer and make handling difficult. The prior art discloses several attempts to overcome this difficulty. The use of sequestering agents, such as sodium hexametaphosphate, has proved unsatisfactory. Other methods involve heat treatment under controlled conditions to form various compounds which retain the metal impurities in solution, but such methods have the disadvantage of tieing up susbstantial amounts of polyphosphate. For example, it has been found that when more than about 2% of Al.sub.2 O.sub.3 and Fe.sub.2 O.sub.3 are present, the availability of the phosphate is substantially reduced due to the formation of iron and aluminum pyrophosphates. Still another prior art method for overcoming the aforementioned problem comprises neutralizing the crude phosphoric acid at elevated pressure and temperature to cause the metal compounds to precipitate in a form which is readily filtered from the resulting polyphosphate solution. This process has the disadvantage of requiring pressure resistant equipment which adds to the cost.
U.S. Pat. No. 3,464,808 to Kearns relates to manufacture of ammonium polyphosphates from wet process phosphoric acid. Phosphoric acid having P.sub.2 O.sub.5 content of between about 55 and 65% is reacted with ammonia under conditions that simultaneously neutralize and molecularly dehydrate the acid whereby at least 20% of the orthophosphate is converted to non-orthophosphate (polyphosphate) and the resulting ammonium polyphosphate possesses self-sequestering properties. The patent features supplying a stream of ammonia to a jet reactor, supplying a stream of concentrated phosphoric acid to said jet reactor, and contacting said ammonia and said acid in said reactor at temperatures of 450.degree.F. to 650.degree.F. for a period of less than one second to form molten droplets of ammonium polyphosphate. The molten product is subsequently quenched, e.g., by dissolving it in an aqueous system and further neutralizing the reaction product. Unfortunately, this process does not produce a product high in polyphosphate unless the wet-process phosphoric acid is preheated, and then the reaction mixture becomes so hot that it is highly corrosive. Generally, the reaction vessel is completely corroded within a few weeks.
U.S. Pat. No. 3,677,734 to Carroll et al. discloses that ammonium polyphosphates having a major portion of the P.sub.2 O.sub.5 content thereof in the form of condensed phosphates, are prepared by ammoniating wet-process phosphoric acid with ammonia in a multi-stage operation under specified conditions. The patent features contacting the wet-process phosphoric acid having a P.sub.2 O.sub.5 content of 50 to 65% by weight with impinging ammonia gas at a gas stream velocity of 1,000 to 2,500 feet per second, at an NH.sub.3 :H.sub.3 PO.sub.4 mol ratio of 0.4 to 0.8, a temperature of 375.degree. to 425.degree.F., a pressure of 15 to 75 psi and a time of 0.1 to 4 minutes to form ammonium phosphate product and steam; removing steam from the product; and contacting the ammonium phosphate with excess impinging ammonia at a velocity of 1,000 to 2,500 feet per second, a temperature of 450.degree. to 500.degree.F., a pressure of 30 to 200 psi, and a time of 5 seconds to less than 2 minutes to form ammonium polyphosphate wherein at least 50% of the P.sub.2 O.sub.5 is in polyphosphate form. This process produces an improved product; however, it would be highly desirable to produce a similar product in a single-stage operation instead of a multistage operation.
U.S. Pat. No. 3,492,087 to Moore et al. discloses a process wherein relatively pure ammonium polyphosphate fertilizer solution is prepared by introducing preheated ammonia into wet process phosphoric acid and heating the mixture to at least 205.degree.C. to produce an anhydrous molten mass which is then dissolved in an aqueous medium. Filterable solids are separated to produce a clear ammonium polyphosphate solution. The ammoniation is preferably conducted in two stages, wherein preheated anhydrous ammonia is introduced into the second stage and the unabsorbed ammonia therefrom is used to partially neutralize the acid in the first stage.
Although the disclosure of U.S. Pat. No. 3,492,087 is an important contribution to this art, research studies have been continued in an effort to develop an improved process which would obviate the remaining three main problems of direct ammoniation technology, i.e., formation of relatively large quantities of insoluble iron and aluminum ammonium phosphates, severe corrosion of the reactor, and insufficient heat from the direct ammoniation to produce a product high in polyphosphate content. Clearly, it would be highly desirable to define a direct ammoniation procedure that would substantially avoid the precipitation of solids and thereby eliminate the costly filtration step of the above-described patented process.